The present invention relates to the design of photomasks (xe2x80x9cmasksxe2x80x9d) for use in lithography, and more particularly, to the use of a hybrid mask which provides for the formation of both phase-shifted and non-phase-shifted features with a single exposure.
The present invention also relates to the use of such a mask in a lithographic apparatus, comprising for example:
a radiation system for supplying a projection beam of radiation;
a mask table for holding the mask;
a substrate table for holding a substrate; and
a projection system for projecting at least part of a pattern on the mask onto a target portion of the substrate.
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a mask pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Although specific reference may be made in this text to the use of lithographic apparatus and masks in the manufacture of ICs, it should be explicitly understood that such apparatus and masks have many other possible applications. For example, they may be used in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).
U.S. Pat. No. 5,340,700 (incorporated herein by reference) describes a method of printing sub-resolution features defined by image decomposition. More specifically, the method discloses first decomposing the sub-resolution features into much larger features such that the feature edges are separated far enough from each other so that the aerial images of the feature edges are xe2x80x9cnon-correlatedxe2x80x9d to one another. In other words, the edges are optically xe2x80x9cisolated.xe2x80x9d By exposing such a decomposed mask with pre-determined multiple exposure steps, it was shown that near-half-wavelength contact hole features can be well defined. For printing line features, the method of U.S. Pat. No. 5,340,700 utilizes negative-acting photoresist, because as the negative-acting photoresist has inherently poorer resolution, the multiple exposure, image de-composition method is best suited for printing contact hole features. Although it has very high printed resolution potential, the method disclosed in U.S. Pat. No. 5,340,700 has not yet been widely adopted in the industry, mainly due to the relative complexity of decomposing the images. Moreover, any method utilizing multiple exposure masking steps negatively impacts the throughput of the lithographic exposure apparatus.
In recent years, the phase-shift mask (xe2x80x9cPSMxe2x80x9d) has been gradually accepted by the industry as a viable alternative for sub-exposure-wavelength manufacturing. Since the early design (Levenson et al., 1982), many forms of PSM have been developed over the years. Of those, two fundamental forms of PSM have been investigated the most, namely alternating PSM (xe2x80x9caltPSMxe2x80x9d) as illustrated in FIGS. 1(a) and 1(b) and attenuated PSM (xe2x80x9cattPSMxe2x80x9d) as illustrated in FIG. 2.
From the image formation point of view, for approximately a 1:1 ratio of line:space features, altPSM eliminates the 0th diffraction order and forms the image pattern with two beams, namely +/xe2x88x921st diffraction orders. This type of PSM is also referred to as xe2x80x9cstrongxe2x80x9d PSM. xe2x80x9cWeakxe2x80x9d PSM refers to the existence of a 0th diffraction order component for image formation. The stronger the PSM, the smaller the 0th diffraction order component, and vice versa. In theory, altPSM can form image patterns at double the original spatial frequencies. Hence, pattern resolution can be twice as fine. AltPSM is often referred to as xe2x80x9cstrongxe2x80x9d PSM since it offers the best resolution improvement potential. To achieve the highest possible resolution potential for altPSM, it is common to use relatively coherent illumination. However, this intensifies the already very strong optical proximity effect (OPE). Such a strong OPE essentially limits the use of altPSM for a wide range of feature pitches, mainly due to the difficulty of getting critical dimension (CD) variations under control.
An altPSM can be either a clear field or a dark field mask, as shown in FIGS. 1(a) and 1(b). As shown in both FIGS. 1(a) and 1(b), the features 10 are disposed between 0-xcfx80 phase-shifting pairs formed by elements 14 and 12. In the Figures, S indicates the mask substrate (carrier plate)xe2x80x94e.g. made of quartz or CaF2xe2x80x94and C indicates chrome areas. Currently, there are significant obstacles for implementing clear field altPSMs for mask designs; one is the near-impossible task of assigning 0-xcfx80 phase pairs without creating phase conflicts: often, many design compromises must be made or the effectiveness of resolution enhancement has to be substantially reduced. Moreover, for the phase transitions (from 0 to xcfx80 or vice versa) that occur in unintended areas, undesirable resist patterns will be formed. In order to correct the foregoing, either highly complicated phase transitions or resort to an additional exposure mask for eliminating these undesirable resist patterns must be utilized (see, e.g. U.S. Pat. Nos. 5,573,890 and 5,858,580, incorporated herein by reference).
Of late, substantial attention has been given to dark field implementations. This is partially due to the fact that the phase assignment issue is easier to deal with and the unwanted resist patterns can effectively be xe2x80x9ctrimmedxe2x80x9d by a second exposure mask. In order to make use of dark field altPSM for line/space patterns, two exposure masks are requiredxe2x80x94one is a non-phase-shifted chrome mask and the other exposure mask is a dark field altPSM. For a typical implementation, the altPSM is only to be used for xe2x80x9ctrimmingxe2x80x9d the width of the gate electrode features after the chrome mask exposure has been made. Since the binary chrome mask is a clear field type, this exposure mask also serves to eliminate unwanted resist patterns formed by unintended phase transitions during the altPSM exposure. An example of the foregoing is shown in FIGS. 3(a) and 3(b), which illustrate resulting resist patterns. Specifically, FIGS. 3(a) and 3(b) illustrate the optical proximity effect of the dark field altPSM, and that the aerial image intensity (I) can be influenced by the size and proximity of the 0-xcfx80 window pairs, illustrated as elements 20 and 22.
Although the complexity of phase assignment can be greatly reduced with dark field altPSM, the issue of overly strong optical proximity effects (OPE) can still severely limit the control of critical dimension for gate features. When the lengths and widths of 0-xcfx80 window pairs are in sub-micron dimensions, they can be susceptible to strong OPE. The proximity of neighboring 0-xcfx80 pairs, and 0-xcfx80-0 or xcfx80-0-xcfx80 windows, also causes strong OPE, as shown in FIGS. 3(a) and 3(b). Another form of OPE is the corner rounding effect. Due to the use of relatively coherent illumination, electric fields are much stronger in the corners. If the length of the 0-xcfx80 window pair is not sufficiently great, the printed window patterns become oval shaped. Thus, the xe2x80x9ctrimmedxe2x80x9d gate features will then become curved, as shown in FIGS. 5(a)-5(d). Although aggressive optical proximity correction (OPC) could be applied, the added mask design complexity and the still strong residual OPE imposes a fundamental limit on minimum feature pitch. Further, as mentioned above, the two required exposures disadvantageously reduce the throughput of the lithographic exposure apparatus.
AttPSM has been customarily described as xe2x80x9cweakxe2x80x9d PSM. For KrF exposure wavelength applications, the commercially available attenuated mask blanks have a range of 5% to 8% transmission of the actinic wavelength. AttPSM does allow the 0th diffraction order to form image patterns, so the resolution enhancement potential is not as good as altPSM. On the other hand, it has a lesser degree of OPE. In addition, the mask design is much less complicated as compared to altPSM, since the design of 5%-8% attPSM is no different from the binary chrome mask when it comes to making line/space style mask patterns. In order to obtain better resolution, it has been observed that the use of higher % transmission is necessary, since the magnitude of the 0th diffraction order is then further reduced. Hence, the best possible resolution enhancement for attPSM is one that has no actinic wavelength attenuation, such as a chromeless phase-shifted mask (xe2x80x9cCLMxe2x80x9d) as shown in FIG. 4.
The present inventors disclosed in European Patent Application EP 0 980 542 (incorporated herein by reference) that CLM could be used in conjunction with incoherent illumination ("sgr" greater than 0.6) for better CD control. Using advanced off-axis illumination (xe2x80x9cOAIxe2x80x9d), the patterning performance can be still further improved. It has been shown that halftone CLM with OAI can print feature widths one-quarter of the exposure wavelength in size. However, the down side for such a high % transmission is the leak-through of the actinic wavelength during the exposure. This occurs when the two phase-edges are separated far enough apart that destructive interference cannot be formed. The resist will then be exposed to cause undesirable patterns. To prevent this, xe2x80x9cwide-openxe2x80x9d areas must either be blanked by the opaque chrome or with periodic phase edge patterns (halftoning patterns) to ensure continuous dark interference for the entire pattern area. In order to avoid dealing with complex proximity effects, currently both methods have been restricted to very large feature patterns.
Accordingly, there remains a need for a photomask that allows for the printing of high resolution xe2x80x9ccriticalxe2x80x9d features, while simultaneously allowing for the printing of low resolution xe2x80x9cnon-criticalxe2x80x9d features, so as to reduce the overall need for optical proximity correction techniques and provide improved CD control for xe2x80x9ccriticalxe2x80x9d features.
In an effort to satisfy the aforementioned needs, it is an object of the present invention to provide a xe2x80x9chybridxe2x80x9d mask, wherein only the critical features of the desired device to be imaged (e.g. integrated circuit) are formed by strongly phase-shifted mask patterns, and the other xe2x80x9cnon-criticalxe2x80x9d features are formed by either weak-phase-shifted or non-phase-shifted chrome mask patterns.
More specifically, the present invention relates to a xe2x80x9chybridxe2x80x9d mask, wherein the high-resolution fine mask features, such as the gate electrodes of integrated circuits, are designed with a high % (e.g.  greater than 10%) transmission attPSM (or a CLM), while the coarser features, such as local interconnects and contact landing pads, are designed with either non-phase-shifting chrome or the standard (e.g. 5-8%) low transmission. Importantly, both the fine mask features and the coarser features are formed simultaneously by a single exposure of the hybrid mask of the present invention.
The present invention also relates to a method of forming the hybrid mask. The method includes the steps of forming at least one non-critical feature on the mask utilizing one of a low-transmission phase-shift mask (pattern) or a non-phase shifting mask (pattern), and forming at least one critical feature on the mask utilizing a high-transmission phase-shift mask (pattern).
As described in further detail below, the present invention provides significant advantages over the prior art. Most importantly, the hybrid photomask of the present invention allows for the printing of high resolution xe2x80x9ccriticalxe2x80x9d features, while simultaneously allowing for the printing of low resolution xe2x80x9cnon-criticalxe2x80x9d features, thereby reducing the overall need for optical proximity correction techniques and providing for improved CD control for xe2x80x9ccriticalxe2x80x9d features.
In addition, because the hybrid mask of the present invention requires only one exposure to form the desired features, the overall throughput of the manufacturing process is advantageously increased by eliminating the need for performing a double exposure and alignment.
Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention.
The invention itself, together with further objects and advantages, can be better understood by reference to the following detailed description and the accompanying schematic drawings.